Evaluation of Electroless Ni-Au Finishes for Wirebonding on PWBs

摘要

Direct chip attach techniques are becoming more prevalent among higher end printed wiring board (PWB) assemblies. Chip-On-Board (COB) packaging with wirebonds, and flip chip packaging both require advanced metal finishes for the printed wiring boards that are used to achieve the higher levels of integration in today's electronic assemblies. This metallization needs to reliably withstand the multiple temperature cycles inherent to the assembly process. This paper examines how wirebondability is affected by variations in the surface finish thickness. At the Johns Hopkins University Applied Physics Laboratory, the finish metallization employed for COB applications is Cu-Ni-Au. The starting copper layer is the standard half ounce foil (18 microns thick) that is bonded to thepolyimide PWB laminate from the manufacturer. After typical PWB processing has been completed, electroless nickel and electroless gold layers are deposited on the patterned copper traces. Common plating thicknesses for Ni can range from 2.5 to 10 microns and for Au the range is 0.5 to 1.5 microns. These ranges are too wide and poor wirebondability has been observed on boards with thin plating. Excessively thick plating will increase the cost of the board with no benefit to bonding. Design of experiments and statistical analysis techniques were used to examine the effects of the nickel and gold film thicknesses, assembly temperatures, temperature cycles, and time at temperature on the initial wirebondability of the board and final reliability of the wirebonds. Failure analysis was also performed on the boards to determine the cause of the poor wirebondability and wirebond failures. The results show that optimum nickel and gold thickness can be determined for various applications.